Divalent carbon ligands

We will call such compounds free carbenes to distinguish them from metal complexes possessing divalent carbon ligands. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

As a result, the reactivity of these metal-NHC compounds is also unique. They prove to be rather resistant toward attack by nucleophiles or electrophiles at the divalent carbon atom. Additionally, theoretical calculations and experimental investigations agree that the ligand dissociation energy for an NHC is higher than for a phosphine. ... 

Abstract The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent carbon(O) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The bonding situation is best described in terms of L C L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(ll) compounds (carbenes) through the number of lone electron pairs. The stmctures and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds. 

The theoretical section introduced divalent carbon(O) compounds as molecules CL2 where the ligand L is a ct donor. In principle this task can be fulfilled by various neutral group 15 compounds (N2, nitriles, amines, phosphanes, arsanes, etc.), neutral group 16 compounds (sulfides, selenides, etc.) as well as by divalent C(II) with a free pair of electrons at a carbon atom, such as isonitriles, NHCs, carbenes, CO, ylides, etc. The neutral and isolable compound C2PPh3 [14,15] may also serve as a donor L to stabilize a carbon atom. 

Table 2 shows those double ylides or related compounds for which transition metal complexes A CL2 or A CLL are known. A more detailed description of the divalent carbon(O) compounds in Table 2 which act as ligands in transition metal chemistry are given in the following sections. 

The number of theoretical investigations of transition metal complexes with carbodiphosphoranes and related divalent carbon(O) ligands is rather small. Quantum chemical calculations of the nickel complexes (CO) Ni-C(PPh3)2 with n = 2, 3 have been pubhshed together with experimental work which describes the S3mthesis and X-ray structure analyses of the compounds [107]. The first systematic... 

In this section we review experimental work about transition metal complexes bearing divalent carbon(O) ligands [M]-CL2. 

Transition Metal Complexes with Heterocyclic and Cyclic Divalent Carbon(O) Compounds as Ligands... 

The divalent carbon(O) atom in L C L has two lone electron pairs which makes CL2 a particular class of ligands which may bind as a bidentate Lewis base to one and to two monodentate Lewis acids. The nature of the ligand L determines whether a divalent carbon atom behaves as a carbone or as a carbene. The newly gained insight into the electronic structure of carbones opens a large field for theoretical and experimental research. 

Carbenes are speties with a divalent carbon atom with various substituents and a lone pair of electrons. Classic carbene gold complexes were synthesized in the coordination sphere of the gold atom, addition of amines or alcohols to the coordinated isocyanide ligands. N-Heterocyclic carbenes (Arduengo s carbenes)... 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

For a long time, carbenes, neutral carbon species with a divalent carbon atom bearing six valence electrons, were considered to be too reactive to be isolated [1]. As a consequence, many chemists hesitated to make use of these compounds, especially as spectator ligands for transition metal chemistry. However, whereas the majority of carbenes are short-lived reactive intermediates, this picture does not hold for N-heterocyclic carbenes [2],... 

The nickel(II)-catalyzed polymerization of isocyanides proceeds relatively fast, a remarkable observation given the steric crowding that is introduced upon formation of the polymer chain. The driving force for the reaction is the conversion of a formally divalent carbon in the monomer into a tetravalent carbon in the polymer, yielding a heat of polymerization of 81.4 kJ moD1.169 For this polymerization reaction, a merry-go-round mechanism has been proposed. Upon mixing of the isocyanides with the Ni(II) catalyst, a square-planar complex is formed (Scheme 7), which in some occasions can be isolated when bulky isocyanides are used. Subsequent attack by a nucleophile on one of the isocyanide ligands is... 

Figure 14 Whitesides complex of a divalent sulphonamide ligand with a carbonic anhydrase dimer, in this case, there is no reference system, because /Cjntra was measured directiy using a competitive ligand to displace one of the sulphonamides. (CA)2 structure taken from PDB (3PJJ).

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